1. Field of the Invention
The present invention relates to an image forming apparatus and specifically relates to the temperature control of a thermal fixing device After developing a latent image optically formed on the photo sensitive and pressure sensitive recording medium on the developer sheet by pressing the recording medium and developer sheet superimposed on each other, the image forming apparatus of this invention promotively forms the image by heat fixing in the thermal fixing device.
2. Description of Related Art
As described in U.S. Pat. No. 4,827,356, an image forming apparatus using a photo sensitive and pressure sensitive recording medium and a developer sheet has a thermal fixing means, a temperature detection means to detect the temperature in the thermal fixing means, and control means to control the temperature of the thermal fixing means based on the signal detected by the temperature detection means. The image forming apparatus is controlled so as to keep the temperature in the thermal fixing means substantially at a preset temperature.
However, the temperature in the image forming apparatus rises when energized for a long time because the thermal fixing means is installed in the image forming apparatus. As a result, the sensitivity of the photo sensitive and pressure sensitive recording medium in the apparatus changes. Thus, the following problem exists when the image is formed in the above-mentioned situation. There is a difference between the color of the output image formed based on the above-mentioned condition and the color of the image output at the normal temperature condition. A similar problem arises in other image forming apparatus as well in that continued exposure to high temperature can have a degrading effect on other recording media such as ordinary paper.
Furthermore, temperature control of the fixing means is typically based upon duty control of a fixed current heating device. The current value is chosen to optimize the control characteristic for steady state operation, e.g., to minimize fluctuation about a target temperature inside the fixing device and to avoid temperature rises above a certain level outside the fixing device. However, an optimum current value for steady state operation is not optimum for a start up condition when it is desirable to warm up the fixing device to its operating temperature with minimum delay. Thus, a warm up time is undesirably lengthened.